In recent years, zirconium phosphate-based inorganic ion exchangers have been utilized in various applications by making use of their characteristics. With regard to the zirconium phosphate-based inorganic ion exchangers, there are amorphous ones, crystalline ones having a 2-dimensional layer structure, and crystalline ones having a 3-dimensional network structure. Among them, hexagonal zirconium phosphate, which has a 3-dimensional network structure, is excellent in terms of heat resistance, chemical resistance, radiation resistance, low thermal expansion properties, etc., and is applied to the immobilization of radioactive waste, solid electrolytes, gas adsorbing/separating agents, catalysts, antimicrobial agent starting materials, etc.
Various hexagonal zirconium phosphates are known to date. Examples thereof include AxNH4(1−X)Zr2(PO4)3.nH2O (ref. e.g. Patent Publication 1), AZr2(PO4)3.nH2O (ref. e.g. Patent Publication 2), and HnR1−nZr2(PO4)3.mH2O (ref. e.g. Patent Publication 3).
Zirconium phosphates in which the ratio of Zr to P varies are also known. Examples thereof include Na1+4xZr2−x(PO4)3 (ref. e.g. Nonpatent Publication 1), Na1+2xMgxZr2−x(PO4)3 (ref. e.g. Nonpatent Publications 1 and 2), and Na1+xZr2SixP3−xO12 (ref. e.g. Nonpatent Publications 2 and 3).
With regard to a process for synthesizing these hexagonal zirconium phosphates, a calcination method in which synthesis is carried out by mixing starting materials and then calcining the mixture at 1,000° C. or higher using a calcining furnace, etc., a hydrothermal method in which synthesis is carried out by mixing starting materials in water or in a state in which they contain water and then heating under pressure, a wet method in which synthesis is carried out by mixing starting materials in water and then heating at normal pressure, etc. are known.
Among these methods, the calcination method enables zirconium phosphate having an appropriately adjusted P/Zr ratio to be synthesized just by mixing starting materials and heating them at high temperature. However, in the calcination method it is not easy to mix the starting materials uniformly, and it is difficult to get a zirconium phosphate having a homogeneous composition. Furthermore, since it is necessary to carry out grinding and classification after calcination in order to obtain particles, there are problems with quality and productivity. Moreover, it is obviously impossible to synthesize a crystalline zirconium phosphate containing ammonia by the calcination method. On the other hand, the wet method and the hydrothermal method can give homogeneous fine particulate zirconium phosphate, but apart from one having a P/Zr ratio of 1.5, and one having a P/Zr ratio of 2 represented by Formula (3) below, no crystalline zirconium phosphate is known.NH4ZrH(PO4)2  (3)
Silver, copper, zinc, tin, mercury, lead, iron, cobalt, nickel, manganese, arsenic, antimony, bismuth, barium, cadmium, chromium, etc. ions have for a long time been known as metal ions that exhibit antimold properties, antimicrobial properties, and antialgal properties (hereinafter, abbreviated to antimicrobial metal ions). In particular silver ion is widely used as a silver nitrate aqueous solution having a disinfecting action and a sterilizing action. However, many of the above-mentioned metal ions that exhibit antimold properties, antimicrobial properties, or antialgal properties are harmful to the human body; there are various restrictions on the application method, storage method, disposal method, etc., and their applications are also limited.
In order to exhibit antimold properties, antimicrobial properties, and antialgal properties, it is sufficient to apply a trace amount of antimicrobial metal to an application target. Because of this, there have been proposed as antimicrobial agents having antimold properties, antimicrobial properties, and antialgal properties an organic supported antimicrobial agent having an antimicrobial metal ion supported on an ion-exchange resin, a chelate resin, etc. and an inorganic antimicrobial agent having an antimicrobial metal ion supported on a clay mineral, an inorganic ion-exchanger, or a porous body.
With regard to the above-mentioned various types of antimicrobial agents, compared with the organic supported type inorganic antimicrobial agents have the advantages of higher safety, a longer lasting antimicrobial effect and, moreover, excellent heat resistance.
As one of the inorganic antimicrobial agents, an antimicrobial agent in which alkali metal ions such as sodium ions in a clay mineral such as montmorillonite or zeolite are ion-exchanged with silver ions is known. Since the skeleton structure of the clay mineral itself has poor acid resistance, silver ions are easily leached in, for example, an acidic solution, and the antimicrobial effect does not last long.
Furthermore, since silver ions are unstable toward exposure to heat and light and are easily reduced to metallic silver, there are problems with long-term stability, such as coloration being caused.
In order to increase the silver ion stability, there is one in which silver ions and ammonium ions are supported on a zeolite by ion-exchanging so that they coexist. However, the prevention of coloration does not reach a practical level even in this system, and a fundamental solution to the problem has yet to be found.
Furthermore, as another inorganic antimicrobial agent, there is an antimicrobial agent having an antimicrobial metal supported on an adsorptive active carbon. However, in this agent since a soluble antimicrobial metal salt is only physically adsorbed or attached, when contacted with moisture the antimicrobial metal ion is rapidly leached, and the antimicrobial effect does not last long.
Recently, an antimicrobial agent having antimicrobial metal ions supported on a special zirconium phosphate salt has been proposed. For example, one represented by Formula (4) below is known (ref. e.g. Patent Publication 4).M1M2xHyAz(PO4)2.nH2O  (4)(In Formula (4), M1 is one type selected from 4-valent metals, M2 is one type selected from silver, copper, zinc, tin, mercury, lead, iron, cobalt, nickel, manganese, arsenic, antimony, bismuth, barium, cadmium, and chromium, A is one type selected from alkali metal ions and alkaline earth metal ions, n is a value satisfying 0≦n≦6, and x, y, and z are values satisfying each of 0<(I)×(x)<2, 0<y<2.0<z<0.5, and (I)×(x)+y+z=2, provided that I is the valence of M2.)
This antimicrobial agent is known as a material that is chemically and physically stable and exhibits antimold and antimicrobial properties for a long period of time. However, when it is kneaded with a synthetic resin such as nylon, the entire resin might be colored, the processability is poor due to the particle size, and it cannot be used as a product.
(Patent Publication 1) JP-A-6-48713 (JP-A denotes a Japanese unexamined patent application publication.)
(Patent Publication 2) JP-A-5-17112
(Patent Publication 3) JP-A-60-239313
(Patent Publication 4) JP-A-3-83906
(Nonpatent Publication 1) C. JAGER and three others, ‘31P and 29Si NMR Investigatios of the Structure of NASICON-Strukturtyps’, Expermentelle Technik der Physik, 1988, Vol. 36, No. 4/5, p 339-348
(Nonpatent Publication 2) C. JAGER and two others ‘31P MAS NMR STUDY OF THE NASICON SYSTEM Na1+4yZr2−y(PO4)3’. Chemical Physics Letters, 1988, Vol. 150, No. 6, p 503-505
(Nonpatent Publication 3) H. Y-P HONG, ‘CRYSTAL STRUCTURE AND CRYSTAL CHEMISTRY IN THE SYSTEM Na1+xZr2SixP3−x—O12’, Mat. Res. Bull., Vol. 11, p 173-182